1. Field of the Invention
The present invention relates to a voltage monitoring circuit for monitoring the voltage used for writing data to or erasing it from a semiconductor storage and, more particularly, to a voltage monitoring circuit intended for a semiconductor storage incorporated in a memory card or the like.
2. Description of the Related Art
Cards in which information can be recorded have been used in a variety of fields. The cards are available as magnetic cards for retaining necessary information in the form of magnetic information or as memory cards designed to store necessary information in semiconductor storages. The memory cards come in two types: one type has a built-in semiconductor storage, and the other type incorporates a program memory or the like for implementing required processing by accessing a central processing unit or other desired processing; the latter type is known as an IC card. Semiconductor devices built in the memory cards include dynamic random access memories or static random access memories that permit read and write (hereinafter referred to as "RAM"), electrically erasable and programmable read only memories (hereinafter referred to as "EEPROM"), and flash memories that permit erasure in block sizes.
Generally, when a user of a memory card tries to use the memory card, the personal identification number of the user is checked to make sure that the person attempting to use it is the authorized user of the memory card. The IC card is available in two types: in one type of IC card, the personal identification number is checked on the card reader/writer side or the host side; in the other type, the check is performed within the IC card. The following describes an example of the checking procedure applied to the type of IC card adapted to check the personal identification number within the IC card.
First, a personal identification number is entered through an input circuit provided on the surface of a card medium of the IC card. The entered personal identification number is checked against the personal identification number that has been stored in the IC card beforehand. When the check result indicates agreement, the use of the IC card is authorized to proceed to the subsequent processing such as referring to a transaction history or other information that has already been stored in the IC card. If the check result indicates disagreement, then the personal identification number is entered again to repeat the check. If the check result indicates disagreement for a predetermined number of times, e.g., three in succession, then the subsequent processing of the IC card is disabled and the use of the IC card itself is prohibited at the same time. The number of disagreements revealed by the check is stored and updated in a built-in semiconductor storage as the number of error counts each time the check result shows the disagreement. Hence, when a error count number reaches the predetermined number, e.g. three, the use of the IC card itself will be prohibited.
To check the personal identification number for the IC card by using the card reader/writer, the IC card is inserted in the card reader/writer and the personal identification number is entered through the input circuit provided on the card reader/writer. The rest of the procedure is identical to that mentioned above.
Such a function prevents unauthorized use of an IC card obtained by a third person who has obtained the IC card by theft or the like unless the third person knows the authorized personal identification number of the IC card. The personal identification number is composed, for example, of a combination of a plurality of characters or numerals so that it cannot be guessed correctly within a predetermined number of attempts. Thus, the security of IC cards is maintained.
As described above, the error count number is written to the semiconductor storage built in the IC card. For instance, to write data to or erase it from the IC card, a voltage, e.g., a high voltage of about 20 volts for an EEPROM, is required. If the supply of such a voltage to the semiconductor storage is prohibited due to damage to a source generating the voltage required for writing or erasing or due to disconnected wiring for supplying the voltage required for writing or erasing to the semiconductor storage, then writing or erasing the error count number is disabled. This applies not only to the IC cards but also to the memory cards that have built-in semiconductor storages requiring such a special voltage.
If an unauthorized third party tries entering personal identification numbers to use a card which has been reworked so that the supply of the voltage required for writing or erasing to a semiconductor storage is disabled, then the error count number is no longer written to or updated in the semiconductor storage. This enables the check of a personal identification number to be performed for a number of times exceeding a predetermined number. This presents a problem in that a third party may eventually determine a correct personal identification number, and as a result, unauthorized use of an IC card or a memory card will be permitted.